Large scale patterned growth of aligned one-dimensional nanostructures

ABSTRACT

A method of making nanostructures using a self-assembled monolayer of organic spheres is disclosed. The nanostructures include bowl-shaped structures and patterned elongated nanostructures. A bowl-shaped nanostructure with a nanorod grown from a conductive substrate through the bowl-shaped nanostructure may be configured as a field emitter or a vertical field effect transistor. A method of separating nanoparticles of a desired size employs an array of bowl-shaped structures.

CROSS-REFERENCE TO A RELATED NON-PROVISIONAL PATENT APPLICATION

The present application claims priority on U.S. Provisional patentapplication Ser. No. 12/028,218, filed Feb. 8, 2008 and on Ser. No.11/010,178, filed Dec. 10, 2004, the entirety of both of which isincorporated herein by reference.

CROSS-REFERENCE TO A RELATED PROVISIONAL PATENT APPLICATION

The present application claims priority on U.S. Provisional PatentApplication Ser. No. 60/528,740, filed Dec. 11, 2003, the entirety ofwhich is incorporated herein by reference.

STATEMENT REGARDING GOVERNMENT RIGHTS

This invention was made with Government support under Contract No.DMR-9733160, awarded by the National Science Foundation, and ContractNo. ARO DAAD 19-01-0603, awarded by the U.S. Army, the United StatesGovernment therefore has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to nanostructures and, more specificallyto patterned nanostructures.

2. Description of the Prior Art

Binary semiconducting oxides often have distinctive properties and canbe used as transparent conducting oxide (TCO) materials and gas sensors.Current studies of semiconducting oxides have been focused ontwo-dimensional films and zero-dimensional nanoparticles. For example,fluorine-doped tin oxide films are used in architectural glassapplications because of their low emissivity for thermal infrared heat.Tin-doped indium oxide (ITO) films can be used for flat panel displays(FPDs) due to their high electrical conductivity and high opticaltransparency; and zinc oxide can be used as an alternative material forITO because of its lower cost and easier etchability. Tin oxidenanoparticles can be used as sensor materials for detecting leakage ofseveral inflammable gases owing to their high sensitivity to low gasconcentrations.

In contrast, investigations of wire-like semiconducting oxide nanostructures can be difficult due to the unavailability of nanowirestructures. Wire-like nano structures have attracted extensive interestover the past decade due to their great potential for addressing somebasic issues about dimensionality and space confined transport phenomenaas well as related applications. In geometrical structures, thesenanostructures can be classified into two main groups: hollow nanotubesand solid nanowires, which have a common characteristic of cylindricalsymmetric cross-sections. Besides nanotubes, many other wire-likenanomaterials, such as carbides, nitrides, compound semiconductors,element semiconductors, and oxide nanowires have been successfullyfabricated.

However, the nanostructures discussed above can have a variety ofdeficiencies. For example, often it is difficult to control thestructure and morphology of many nanostructures. Further, manynanostructures are not defect and/or dislocation free. Thesedeficiencies can cause problems such as, for example, uncontrolledproperties due to uncontrolled structure and morphology, scattering fromdislocations in electric transport applications, and degraded opticalproperties.

Semiconducting oxides, as an important series of materials candidatesfor optoelectronic devices and sensors, have attracted considerableattention in scientific research and technological applications.Recently, quasi-one-dimensional nanostructures for the functionalmaterials have been successfully fabricated by using various approachesincluding thermal evaporation, sol-gel, arc discharge, laser ablationand template-based method. To date, extensive research work has beenfocused on ZnO, which is one of the most useful oxides for optical andsensor applications. Many different morphological ZnO nanostructures,including wires, belts, and rods, etc., have been fabricated.

In ZnO, a combination of the three types of fast growth directions ([2 11 0], [0 1 1 0], and [0001]) and the three area-adjustable facets [2 1 10], [0 1 1 0], and [0001]) of ZnO has resulted in a diverse group ofhierarchical and intricate nanostructures. In addition to non-centralsymmetry, the semiconducting and piezoelectric as well as surfacepolarization characteristics of ZnO make it one of the most excitingoxide nanostructures for investigating nano-scale physical and chemicalproperties. Structural configurations such as piezoelectric nanobelts,nanosprings, and nanorings, etc., are known.

Certain patterned nanostructures have utility in many differentapplications, including electronics, optics and size differentiation ofvarious particles. Unfortunately, such patterned nanostructures may bedifficult to make at a large scale.

Therefore, there is a need for a method of making patternednanostructures at a large scale.

There is also a need for patterned nanostructures.

SUMMARY OF THE INVENTION

The disadvantages of the prior art are overcome by the present inventionwhich, in one aspect, is a nanostructure that includes a substratehaving a top surface and a metal oxide bowl-shaped structure. Thebowl-shaped structure has a bottom end and a maximum diameter in a rangeof 50 nanometers to 4,000 nanometers. The bowl-shaped structure isdisposed on the top surface of the substrate with the bottom endadjacent thereto. The bowl-shaped structure defines a hemispherical voidthat opens away from the top surface of the substrate.

In another aspect, the invention is a method of making a bowl-shapedstructure, in which an organic sphere is placed onto a top surface of asubstrate and a metal oxide is applied to the organic sphere, therebyforming a metal oxide shell. A portion of the metal oxide shell and acorresponding portion of the sphere are removed and any remainingportion of the organic sphere is removed, thereby leaving a bowl-shapedstructure.

In another aspect, the invention is an array of patterned nanostructuresthat include a substrate having a top surface and a plurality of metaloxide nanorods extending upwardly from the top surface. The nanorods aredisposed in an patterned arrangement.

In another aspect, the invention is a method of making patternednanostructures, in which a self-assembled monolayer of organic spheresis placed on a substrate. The organic spheres defining interstitialareas between the organic spheres. Catalyst particles are applied andallowed to settle onto the substrate in the interstitial areas. Theorganic spheres are removed, thereby leaving a patterned arrangement ofcatalyst particles on the substrate. Nanorods are grown from thesubstrate in the interstitial areas.

In another aspect, the invention is a field emitter that includes aconductive substrate, an insulating layer, a bowl-shaped structure, anda nanorod. The insulating layer is disposed on the conductive substrateand defines a first hole therethrough. The bowl-shaped structure extendsupwardly from the insulating layer and defines a second hole insubstantial alignment with the first hole. The bowl-shaped structuredefines a concave void. The nanorod extends upwardly from the conductivesubstrate through the first hole and the second hole into the concavevoid. When a predetermined potential is applied between the bowl-shapedstructure and the rod, electrons are emitted from the rod.

In another aspect, the invention is a method of making a field emitter,in which a catalyst mantle is applied to a substrate, an insulatinglayer is applied to the catalyst mantle, and an organic sphere is placedonto the insulating mantle. A metal oxide is applied to the organicsphere, thereby forming a metal oxide shell. The metal oxide shelldefines an opening to the insulating layer at an area where the organicsphere was in contact with the insulating layer. A portion of the metaloxide shell and a corresponding portion of the sphere are removed. Anyremaining portion of the organic sphere is removed, thereby leaving abowl-shaped structure. A hole is etched into the insulating layerthrough the opening defined by the metal oxide shell. The hole extendsto the catalyst mantle. Heat sufficient to melt a portion of thecatalyst mantle is applied so as to form a catalyst particle inalignment with the hole etched in the insulating layer. An elongatednanostructure is grown from the catalyst particle from the substratethrough the hole etched in the insulating layer into the bowl-shapedstructure.

In another aspect, the invention is a field effect transistor circuitthat includes a first nano-bowl structure and a second nano-bowlstructure in vertical alignment with the first-nanobowl structure. Thefirst nano-bowl structure includes a conductive substrate, an insulatinglayer disposed on the conductive substrate, a nano-bowl disposed on theinsulating layer and an elongated nanostructure extending from theconductive substrate through the insulating layer into the nano-bowl andelectrically isolated from the nano-bowl. Similarly, the secondnano-bowl structure includes a conductive substrate, an insulating layerdisposed on the conductive substrate, a nano-bowl disposed on theinsulating layer and an elongated nanostructure extending from theconductive substrate through the insulating layer into the nano-bowl andelectrically isolated from the nano-bow. When a first electricalpotential is applied to the substrate of the first nano-bowl structure;a second electrical potential, different from the first electricalpotential, is applied to the nano-bowl of the first nano-bowl structure;and a third electrical potential, different from the first electricalpotential, is applied to the substrate of the second nano-bowlstructure, then the substrate of the first nano-bowl structure acts as asource, the nano-bowl of the first nano-bowl structure acts as a gate,and the substrate of the second nano-bowl structure acts as a drain.

In another aspect, the invention is a method of size-sortingnano-particles, in which the nano-particles are placed onto an array ofbowl-shaped structures. Each bowl-shaped structure has a maximumdiameter corresponding to a predetermined desired diameter ofnano-particle. A force is applied to the nano-particles that issufficient to remove all of the nano-particles, except for anynano-particles that have settled into bowl-shaped structures in thearray of bowl-shaped structures.

In yet another aspect, the invention is a method of making a photoniccrystal, in which a patterned array of nanorods is grown on a substrate.A sufficient number of layers of a metal oxide is applied to thenanorods so that a contiguous structure is formed around the nanorods.The contiguous structure defines an array of holes passing therethrough.The holes are spaced apart at a distance that achieves a desired opticalproperty.

These and other aspects of the invention will become apparent from thefollowing description of the preferred embodiments taken in conjunctionwith the following drawings. As would be obvious to one skilled in theart, many variations and modifications of the invention may be effectedwithout departing from the spirit and scope of the novel concepts of thedisclosure.

BRIEF DESCRIPTION OF THE FIGURES OF THE DRAWINGS

FIG. 1 is a top view of a self-assembled monolayer of polystyrenespheres.

FIGS. 2A-2D is a series of cross-sectional drawings showing a method inwhich the monolayer of polystyrene spheres shown in FIG. 1 is used inmaking nano-bowl structures.

FIG. 3 is a is a top view of a self-assembled monolayer of polystyrenespheres and a detail thereof.

FIGS. 4A-4C is a series of cross-sectional drawings showing a method inwhich the monolayer of polystyrene spheres shown in FIG. 1 is used inmaking patterned elongated nanostructures.

FIG. 4D is a top view of a plurality of patterned elongatednanostructures.

FIGS. 5A-5D is a series of cross-sectional drawings showing a method formaking a field emitter.

FIG. 6 is a cross-sectional view of a field effect transistor.

FIGS. 7A-7D is a series of cross-sectional drawings showing a method forseparating nano-particles of a predetermined size.

FIGS. 8A-8G is a series of top views showing a method for making aphotonic crystal.

FIGS. 9A-9E is a series of cross-sectional drawings showing a method forforming a self-assembled monolayer of organic spheres.

FIG. 10 is a cross-sectional view of a tube furnace.

FIGS. 11A-11E is a plurality of micrographs of several embodiments ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

A preferred embodiment of the invention is now described in detail.Referring to the drawings, like numbers indicate like parts throughoutthe views. As used in the description herein and throughout the claims,the following terms take the meanings explicitly associated herein,unless the context clearly dictates otherwise: the meaning of “a,” “an,”and “the” includes plural reference, the meaning of “in” includes “in”and “on.”

In one embodiment of the invention, a monolayer of organic spheres, suchas polystyrene spheres, is self-assembled onto a substrate. As shown inFIG. 1, such a monolayer 100 includes a plurality of spheres 102arranged in an close packing arrangement, such as a hexagonalarrangement. As shown in FIG. 2A, the spheres 102 form a monolayer onthe top surface 202 of a substrate 200 (which could include a singlecrystal of aluminum oxide).

As shown in FIGS. 9A-9E, one way of creating the monolayer 914 on thesubstrate 912 starts with adding a plurality of organic spheres 910 to aliquid 900, such as water, having a surface tension (as a result ofadding a surfactant) that would cause the organic spheres 910 toself-assemble into a substantially ordered monolayer. As shown in FIG.9B, the substrate 912 is placed in the liquid and then gradually drawnout in a direction that is transverse to the top surface of the liquidso that the organic spheres become disposed on the top surface of thesubstrate 912 to form a monolayer 914. Any remaining liquid is thenallowed to dry from the substrate and the organic spheres.

Returning to FIG. 2B, a metal oxide, such as titania, is applied to thespheres 102 to form a metal oxide shell 210 around the spheres 102. Thisis typically done through an atomic layer deposition process.

In the atomic layer deposition process, the organic spheres 102 and thesubstrate 200 are placed in a chamber, which is maintained at apredetermined temperature and a predetermined pressure. Alternatingpulses of a gas phase metal (such as titanium chloride gas) and watervapor are introduced into the chamber until the desired thickness ofmetal oxide has been applied to the organic spheres 102. The chamber ispurged between each pulse with a non-reactive gas, such as nitrogen.

As shown in FIG. 2C, approximately the top half of the metal oxide shell210 and the spheres 102 is removed by an ion milling process. Ionmilling occurs by rotating the substrate 220 and applying an ion beam220 at an angle for a predetermined amount of time. As shown in FIG. 2D,the any remaining portion of the spheres is removed, for example with asolvent such as toluene, thereby leaving an array of bowl-shapedstructures 230. The bowl-shaped structures 230 may then be annealed tocause the metal oxide to crystallize into an anatase.

Each metal oxide bowl-shaped structure 230 has a bottom end and amaximum diameter in a range of 50 nanometers to 4,000 nanometers anddisposed on the top surface 202 of the substrate 200 with the bottom endadjacent to the top surface. Each bowl-shaped structure 230 defines ahemispherical void 232 that opens away from the top surface of thesubstrate.

In another embodiment, an array of patterned nanostructures may be madestarting with a monolayer 100 of organic spheres 102, as shown in FIG.3. As shown in the detail of FIG. 3, the organic spheres defineinterstitial areas 104 between the organic spheres 102. As shown in FIG.4A, catalyst particles 410 (such as gold) are allowed to settle onto thesubstrate 200 in the interstitial areas 104. The catalyst particles caninclude gold. Other examples of catalysts include iron and nickel,depending on the application. One of skill in the art would recognizeother suitable catalysts, depending on the specific embodiment.

The organic spheres 102 are removed, such as with a solvent, and apatterned arrangement of catalyst particles 410 is left on thesubstrate, as shown in FIG. 4B. Metal oxide nanorods 420 (such as zincoxide), or carbon nanotubes, are then grown to extend upwardly from thesubstrate 200 in clusters in the interstitial areas, as shown in FIG.4C. This could be done with a chemical vapor deposition process. Asshown in FIG. 4D, the nanorods 420 are arranged in a patterncorresponding to a projection of the interstitial areas 104 of shapes(such as the spheres 102) onto the substrate 200. One orderedarrangement of the nanorods 420 could correspond to a hexagonal closepacking arrangement.

As shown in FIG. 5D, a field emitter 510, according to one embodiment ofthe invention, includes a conductive substrate 512 that includes acatalyst mantle 514. An insulating layer 518 is disposed on theconductive substrate 512 and defines a first hole 530 therethrough. Abowl-shaped structure 520 extends upwardly from the insulating layer 518and defines a second hole 522 in substantial alignment with the firsthole 530. The bowl-shaped structure 520 defines a concave void 524. Ananorod 540 extends upwardly from the conductive substrate 512 throughthe first hole 530 and the second hole 522 into the concave void 524.When a predetermined potential is applied between the bowl-shapedstructure 520 and the rod 540, the rod will emit electrons. An anode(not shown) may be placed above the rod 540 to absorb the emittedelectrons.

The field emitter 510 may be made, as shown in FIGS. 5A-5D, by applyingthe catalyst mantle 514 (such as a gold layer) to a substrate 512 andapplying the insulating layer 518 to the catalyst mantle 514 and thenmaking a nano-bowl structure as described above.

The second hole 522 is naturally formed as metal oxide is deposited inthe contact area between the organic sphere and the insulating layer518. The metal oxide acts as a mask when etching the first hole 530 intothe insulating layer 518 to the catalyst mantle 514. Heat sufficient tomelt a portion of the catalyst mantle 514 is applied so as to form acatalyst particle 516 in alignment with the hole 530 etched in theinsulating layer. An elongated nanostructure 540 (such as a zinc oxidenanorod or a carbon nanotube) is grown the catalyst particle 516 in themethod for growing patterned nanorods described above.

As shown in FIG. 6, a field effect transistor 600 circuit may be made byvertically aligning a first nano-bowl structure 610 and a secondnano-bowl structure 620. When a first electrical potential is applied tothe substrate 602 of the first nano-bowl structure 610; a secondelectrical potential, different from the first electrical potential, isapplied to the nano-bowl 604 of the first nano-bowl structure 610; and athird electrical potential, different from the first electricalpotential, is applied to the substrate 622 of the second nano-bowlstructure 620, then the substrate 602 of the first nano-bowl structure610 acts as a source, the nano-bowl 604 of the first nano-bowl structure610 acts as a gate, and the substrate 622 of the second nano-bowlstructure 620 acts as a drain. An insulating layer 608 should separatenano-bowl 604 from substrate 622. As would be apparent to those of skillin the electronic arts, this embodiment allows for the creation ofcomplex three-dimensional transistor circuits.

A method of size-sorting nano-particles is shown in FIGS. 7A-7D. Thenano-particles 720 are placed onto an array 710 of bowl-shapedstructures. Each bowl-shaped structure has a maximum diametercorresponding to a predetermined desired diameter of nano-particle. Aforce is applied to the nano-particles 720 that is sufficient to removeall of the nano-particles of undesirable size 724, leaving anynano-particles 722 that have settled into bowl-shaped structures in thearray of bowl-shaped structures.

A method of making a photonic crystal is shown in FIGS. 8A-8G. Apatterned array of nanorods 810 is grown on a substrate 800, asdisclosed above. Layers of a metal oxide 822, 824, 826 and 828 (such astitania) are repeatedly applied to the nanorods 810 untile a contiguousstructure 840 is formed around the nanorods 810. The contiguousstructure 840 defines an array of holes 830 passing therethrough. Theholes 830 are paced apart at a distance that achieves a desired opticalproperty.

The above described embodiments are given as illustrative examples only.It will be readily appreciated that many deviations may be made from thespecific embodiments disclosed in this specification without departingfrom the invention. Accordingly, the scope of the invention is to bedetermined by the claims below rather than being limited to thespecifically described embodiments above.

A tube furnace 1000 of the type that may be used in the chemical vapordeposition process used herein is shown in FIG. 10.

A micrograph 1100 of catalyst particles on a substrate is shown in FIG.11A. A micrograph 1102 of patterned aligned nanorods is shown in FIG.11B. A micrograph 1104 of an array of nanobowls is shown in FIG. 11C. Amicrograph 1106 of an array of nanobowls used in size sortingnanoparticles, with an enlarged detail of a nanoparticles in a nanobowl1108, is shown in FIG. 11D. A micrograph 1110 of an array of nanobowlswith nanorods grown therefrom is shown in FIG. 11E.

Several results of experimental embodiments will now be discussed.

The invention combines the self-assembly based mask technique with thesurface epitaxial approach to grow large-area hexagonal arrays ofaligned ZnO nanorods. This approach opens the possibility of creatingpatterned 1D nanostructures for applications as sensor arrays,piezoelectric antenna arrays, optoelectronic devices, and interconnects.

The synthesis process involves three main steps. The hexagonallypatterned ZnO nanorod arrays are grown onto a single crystal Al₂O₃substrate, on which patterned Au catalyst particles are dispersed.First, a two-dimensional, large-area, self-assembled and orderedmonolayer of sub-micron spheres was formed on a single crystal Al₂O₃substrate. Second, a thin layer of gold particles was deposited onto theself-assembled monolayer; and then the spheres were etched away, leavinga patterned gold catalyst array. Finally, nanorods were grown on thesubstrate using a VLS process. Details on each step are described below.

Monolayer self-assembled arrays of sub-micron spheres: The first stepwas to prepare an ordered monolayer of spheres by self-assembly. Forthis mono-dispersed polystyrene (PS) spheres suspensions were purchasedfrom Duke Scientific Corp. and used as received. The concentration ofthe suspensions was 10% spheres and the diameter of the spheres used inour experiments was 895 nm. For deposition, a 1 cm×1 cm single-crystalsapphire (2 1 1 0) substrate was sonicated for 20 minutes in a 2%Hellmanex II solution followed by a 3 hour anneal in air at 1000° C. toachieve a completely hydrophilic and atomically flat surface. Then, 2 or3 drops of the PS sphere suspension was applied to the surface of thesubstrate. After holding the substrate stationary for 1 minute to obtaingood dispersion of the suspension, the sapphire substrate was thenslowly immersed into deionized water. Once the suspension contacted thewater's surface, a monolayer of PS spheres was observed to immediatelyform, both on the surface of the water and on the surface of thesapphire substrate. To prevent any further additions to the substrate itwas kept immersed. Then, a few drops of 2% dodecylsodiumsulfate solutionwere added to the water to change the surface tension. As a result, themonolayer of PS spheres that remained suspended on the surface of thewater was pushed aside due to the change in the surface tension. Thesubstrate was then removed through the clear area where the surfacetension of the water had been modified by the surfactant; thus, noadditional PS spheres were deposited on the monolayer during its removalfrom the water. A metal frame was used to support the sample above thewater surface while the sample was sonicated to avoid clustering of thePS spheres during drying.

The substrate was 90% covered by the monolayer. The area of a singledomain can reach a few square millimeters.

The self-assembled arrays of polystyrene spheres were then used topattern the catalyst used to guide ZnO growth onto substrate. For thisprocess, gold particles were either sputtered or thermally evaporatedonto the self-assembled monolayer structure; as a result, two differentpatterns were obtained. For the sputtered coatings the high mobility ofthe gold atoms during the sputtering process, resulted in gold coveringevery available area, even beneath the spheres. Therefore, after etchingaway the polystyrene spheres using toluene, this technique produced ahoneycomb-like hexagonal gold pattern. However, by using a thermalevaporator, which provides a line of sight vapor stream, the goldparticles were only deposited onto areas of the substrate that were notshadowed by the polystyrene spheres. After etching away the polystyrenespheres, a highly ordered hexagonal array of gold spots was formed onthe substrate.

Using the patterned catalyst, ZnO nanorods were grown by asolid-liquid-vapor process. The source materials contained equal amounts(by weight) of ZnO powder (0.8 gram) and graphite powder, which was usedto lower the growth temperature. The source materials were then groundtogether and loaded into an alumina boat that was placed at the centerof an alumina tube with the substrate being positioned slightlydownstream from the tube's center. Both ends of the tube were watercooled to achieve a reasonable temperature gradient. A horizontal tubefurnace was used to heat the tube to 950° C. at a rate of 50° C./min andthe temperature was held for 20-30 minutes under a pressure of 300-400mbar at a constant argon flow at 25 sccm. Then the furnace was shut downand cooled to room temperature under a flow of argon.

By changing the growth time the height of the ZnO nanorods could bevaried from a few hundred nanometers to a few micrometers. Most of theZnO nanorods grow perpendicular to the substrate (that is, vertically),but that a few can also grow parallel to the substrate, and have agrowth root from the same catalyst particle that promotes verticalnanorod growth.

Photoluminescence (PL) spectra were acquired from the aligned ZnOnanorods to reveal their collective optical properties. The PLmeasurements were performed at room temperature using a 337 nm N₂ pulsedlaser as the excitation light source. The laser pulse frequency was 15Hz, with 800 ps pulse duration at an average energy of 50 mW. The laserwas incident onto the sample at an angle of 45°, while the detectionangle was defined as the angle between the detector and the directionnormal to the substrate, as shown in the inset in FIG. 5. The emissionspectra were recorded at four different detection angles, 0, 15, 30 and45 degrees. The aligned ZnO nanorods exhibit a peak at 384 nm at θ=0degree. As the detection angle was increased, the luminescence intensitydropped dramatically, which may indicate that the luminescence wasemitted mainly along the axis of the ZnO nanorods. Moreover, theluminescence peak shifted very slightly from 384 nm to 383 nm when thedetection angle increased from 0 to 45 degrees. This may also be causedby the polarization of the emitted light from the aligned nanorods.

The synthesis technique uses a catalyst template produced from aself-assembled monolayer of sub-micron polystyrene spheres that guidesthe VLS growth of ZnO onto a single crystal alumina substrate. Thisapproach opens the possibility of creating patterned 1D nanostructuresfor applications as sensor arrays, piezoelectric antenna arrays,optoelectronic devices, and interconnects.

Nanobowls: Capturing of virus or cells is critically important inbiomedical application, environmental filtering, water purification, andcleaning air pollutions. Due to the small size of the particles, specialsize cups are needed for capturing the virus or cells. In thisdisclosure, we have invented a very economic technique for large-scalefabrication of nanobowls without using lithiography or clean room. Thesize of the nanobowls can be tuned between 50 nm to 10 μm, allowing themto fit to the size of a wide rang of applications. This is a usefultechnology for biomedical research and applications.

The experimental procedure includes four steps. 1. Self-assembly of amonolayer of polystyrene (PS) spheres. A large-area monolayer of PSspheres within a size range from a few hundred nanometers to a fewmicrons can be self-assembled on any substrate with a smooth andhydrophilic surface. 2. Atomic layer deposition (ALD) of amorphous TiO₂thin films. The substrate with a monolayer of PS spheres was placed atthe center of a quartz ALD chamber, which was kept at 80° C. during theentire growth process. Then, pulses of TiCl₄ vapor and H₂O vapor wereintroduced sequentially into the chamber under a vacuum of 4.5 torr. Thepulse duration was 4 second, and the pulses were separated by N₂ purginggas for 10 second. A TiO₂ layer was slowly grown on the surfaces of thePS spheres and the substrate at a growth rate of 0.12 nm/cycle. Thegrowth was terminated after 200 pulse cycles, which produced a uniformamorphous TiO₂ layer of 24 nm in thickness deposited on the surfaces. 3.Ion milling. An ion milling machine that was originally used forpreparing TEM samples was used to remove the top half of the TiO₂ layercoated PS spheres. The ion beam, which was generated by a high-voltageof 5 kV, stroke on the sample surface at a grazing angle of 10° whilethe sample was continuously rotated. After 20-minute milling, the tophalf of the spheres was evenly removed in the case of using 505 nm PSspheres. 4. Toluene etching and annealing. The PS hemispheres that lefton the substrate were etched away by sonicating the substrate in toluenefor 1.5 minutes, resulting in highly-ordered arrays of TiO₂ nano-bowls.The amorphous TiO₂ nano-bowls can be converted into crystallinenano-bowls by annealing. After annealing at 850° C. for 2 hours in air,nano-bowls composed of nanocrystalline anatase TiO₂ were received.

As one of the important functional semiconductors, TiO₂ exhibitspromising applications in solar cell, photocatalytic, and photovoltaictechnology. As described above, robust and highly-ordered anatase TiO₂nano-bowl arrays have been successfully fabricated, which have a largeropen surface area that could significantly increase the efficiency ofsurface related activities. The technique demonstrated can also beapplied to different substrates that have smooth and hydrophilicsurfaces, such as silicon, glass, metals or even polymer. This willbroaden its application in various fields. The nano-bowls could also belifted off from the substrate to form monolayer submicron filters.Moreover, the thickness of the TiO₂ wall can be precisely tuned byvarying the number of ALD cycles, and the size of bowls can be adjustedby using different sized PS spheres as templates. Therefore, the bowlscould also be a good candidate as a size separator and container forfine particles, or even for bio species such as cells if the interiorsurface is coated with proper functional groups.

Field Emission 1D Nanostructures Experimental procedure: First, aconducting substrate, such as silicon wafer or GaN single crystal, iscoated with a thin layer (˜5 nm) of metal catalyst. The metal can begold, nickel, cobalt, iron, copper, and so on, depending on the 1Dnanostructure to be grown. Second, a submicron-sized polystyrene (PS)sphere monolayer is applied onto the substrate through self-assembly.Then, a high-quality TiO₂ layer is evenly coated around the PS spheresand on the substrate by atomic layer deposition (ALD) technique. An ionmilling machine is then used to remove the top half of the TiO₂ coatedPS spheres and the PS hemispheres left on the substrate are etched awayby toluene, resulting in a highly-ordered array of TiO₂ nano-bowls withopen bottoms. Finally, the exposed metal area at the center of thenano-bowls catalyzes the growth of corresponding 1D nanostructuresthrough a Vapor-Liquid-Solid (VLS) process and ended with highly orderedand aligned 1D nanostructures that are separated by TiO₂ nano-bowls.

Field Effect Transistor Arrays Experimental procedure: First, aconducting substrate, such as silicon wafer or GaN single crystal, iscoated with a thin layer (˜5 nm) of metal catalyst. The metal can begold, nickel, cobalt, iron, copper, and so on, depending on the 1Dnanostructure to be grown. Second, a SiO₂ layer is grown onto the metalcatalyst layer to form an insulator layer. Third, a submicron-sizedpolystyrene (PS) sphere monolayer is applied onto the substrate throughself-assembly. Then, a high-quality TiO₂ layer is evenly coated aroundthe PS spheres and on the substrate by atomic layer deposition (ALD)technique. An ion milling machine is then used to remove the top half ofthe TiO₂ coated PS spheres and the PS hemispheres left on the substrateare etched away by toluene, resulting in a highly-ordered array of TiO₂nano-bowls with open bottoms. The SiO₂ at the open bottom area is etchedaway by HF solution so that the metal catalyst beneath the SiO₂ layer isexposed. Finally, the exposed metal catalyzes the growth ofcorresponding 1D nanostructures at the center of each nano-bowl througha Vapor-Liquid-Solid (VLS) process. To form a field emission transistorarray, the conducting substrate that contacts with the 1D nanostructuresis used as the emitter electrode and the TiO₂ bowls that is isolatedfrom the substrate by a SiO₂ layer are used as the gate electrode.

Applications: By growing the aligned ZnO nanorods in the center of thenanobowls, the nanorods' density can be well controlled by varying thesizes of the nanobowls. Thus, the field emission property of the alignedZnO nanorods can be optimized. By introducing an insulating layer suchas SiO₂ between the substrate and catalyst, the nanobowls can beseparated from both the substrate and the ZnO nanorods. Thisconfiguration can be used as a field effect transistor array. When asmall voltage is applied between the substrate and the ZnO nanorodstips, each ZnO nanorod is acting as a field emission tip. Their fieldeffect can be switched on/off by applying another small voltage on thenanobowls array, which is considered as a gate electrode in thisconfiguration. Both of these hierarchical structures are entirely basedon self-assembly technique, which provides a novel technique for quickand large scale fabricating field effect transistor arrays in low costcomparing to lithography. These self-assembled field effect transistorarrays can be used for electron guns and field emission displayers.

What is claimed is:
 1. A method of making patterned nanostructures,comprising the steps of: a. placing a self-assembled monolayer oforganic spheres on a substrate, the organic spheres defininginterstitial areas between the organic spheres, the placing stepincluding: i. adding a plurality of organic spheres to a liquid having asurface tension that would cause the organic spheres to self-assembleinto a substantially ordered monolayer on a top surface of the liquid;ii. placing the substrate in the liquid; iii. drawing the substrate outof the liquid so that the substrate is at an angle that is transverse tothe monolayer so that the organic spheres become disposed on the topsurface of the substrate; and iv. allowing any remaining liquid to dryfrom the substrate and the organic spheres; b. applying catalystparticles to the monolayer and allowing the catalyst particles to settleonto the substrate in the interstitial areas; c. removing the organicspheres, thereby leaving a patterned arrangement of catalyst particleson the substrate; and d. growing nanorods from the substrate in theinterstitial areas.
 2. The method of claim 1, wherein the substratecomprises a single crystal of aluminum oxide.
 3. The method of claim 1,wherein the catalyst particles comprise gold.
 4. The method of claim 1,wherein the removing step comprises etching the organic spheres.
 5. Themethod of claim 4, wherein the etching step comprises applying a solventto the organic spheres.
 6. The method of claim 5, wherein the organicspheres comprise polystyrene and wherein the solvent comprises anorganic solvent.
 7. The method of claim 1, wherein the growing stepcomprises a chemical vapor deposition process.
 8. The method of claim 1,wherein the nanorods comprise zinc oxide.